There have been various known techniques of restraining occupants of motorcycles with airbag apparatuses. One of the known techniques is such that, when a motorcycle collides head-on, an airbag housed in a casing mounted to the body frame is inflated by expansion gas to restrain an occupant (e.g., refer to Japanese Unexamined Patent Application Publication No. 2002-137777). This technique presents a possibility of ensuring a wide restraining area of the airbag, which however needs to increase the size of the airbag correspondingly. This type of airbag therefore requires a technique effective in reliably restraining an occupant with the airbag even with a compact airbag.